This invention relates generally to capacitors and, more particularly, to power capacitors with improved electrical stress capability.
It has been the widespread commercial practice to use aluminum foil electrodes of equal widths in high voltage capacitors used for AC power factor correction or DC applications such as energy storage and discharge or filtering. Because of width tolerances in rolls of aluminum foil and manufacturing tolerances in winding the aluminum foil electrodes, with dielectric spacers, into a convolute capacitor section, the foil electrode edges are not perfectly aligned from one end to the other. It is not considered practical to exercise the degree of care that would be necessary to assure perfect alignment. This results in an electrode system in which the foil edges are offset to some extent. Normally at one edge of the capacitor a first foil extends laterally outward less far than the second foil. At the other side of the capacitor the second foil extends laterally outward less far than the first. In operation, the voltage stress is relatively high at the edge of the recessed foil. Thus, there are points of relatively high stress at each edge of the capacitor. The higher voltage stress results in lower breakdown voltage and lower partial discharge inception voltage than would be achieved if the foils were perfectly aligned.
Two significant developments in power capacitor structures are those disclosed in Yagitani U.S. Pat. No. 3,857,073, Dec. 24, 1974, and Yagitani et al. published Japanese Patent Applications 28516/74 and 34141/74, both of Mar. 11, 1974. The subject matter of said Japanese published applications is included in U.S. application Ser. No. 724,195 filed Sept. 17, 1976, assigned to Kabushiki Kaisha Shizuki Denki Seisakusho, now abandoned. Said published Japanese applications were laid open on Sept. 25, 1975.
In the Yagitani U.S. patent is disclosed a capacitor comprising a layer of capacitor grade paper and a layer of film, particularly polypropylene, as a composite dielectric between two electrode foils of which the foil immediately adjacent the paper is narrower at both edges than the foil immediately adjacent the film. Such structures, impregnated with a dielectric fluid, have been found to compare very favorably in over-voltage tests with otherwise like capacitor structures in which the foil electrodes are like dimensioned and intended to be aligned but subject to normal manufacturing variance from perfect alignment. The virtue of this arrangement is that the narrower electrode is next to the paper layer at both edges which eases the stress problem compared with the case where the electrodes are of the same width and intended to be aligned, but not actually aligned due to manufacturing variances.
In the pending application of Yagatani et al. a further improved electrode foil arrangement is disclosed. Namely, one in which one foil is not only offset from the other at each of its edges, but that narrower foil also has its edge made rounded and smooth such as by folding over the lateral extreme portions of the foil. It is found improvement in over-voltage characteristics results as compared to otherwise like capacitors in which the narrower foil has straight cut edges.
Capacitor dielectrics include various capacitor grade papers and various synthetic plastics in the form of films of polymeric hydrocarbon materials predominantly including members of the polyolefin family of which polypropylene is most widely used in power capacitors. All paper, all film, and combinations of paper and film have been used. As a dielectric liquid impregnant, polychlorinated biphenyls (PCB's) have been most commonly used. Such fluids are today being discontinued because they are considered to be environmental pollutants. Various alternate fluids are now being used and others are under consideration. Some of these appear promising as general replacements for PCB's. However, their electrical properties are not identical to PCB's and the extent of their use and testing has been so limited as compared with that of PCB's that complete assurance of satisfactory performance over the long term is not conclusively available. Therefore, capacitor designers and manufacturers are faced with a basic change in one element, the dielectric liquid, in a tried and proven system which poses a desire for further improvements in other aspects of capacitor design such as can provide the greatest margin for safe, reliable operation, with good over-voltage handling capability, with long life.
The present invention results in large part from a fuller understanding of the mechanism of over-voltage breakdown in relation to electrode arrangement, constituents of the solid dielectric material, and characteristics of the fluid impregnant which results in novel capacitor structures and gives the capacitor designer greater ability to make sound choices for the various elements of the system as will assure good reliable operation.